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Aero engine bearing concept design

Aero engines are the heart of aircraft, and their performance, safety and reliability directly determine the performance, life and reliability of aircraft, and bearing performance is one of the important factors affecting the life and reliability of aero engines.

 

Aero engine bearings have harsh working conditions, especially spindle bearings, which need to meet the requirements of high temperature, high speed, stable operation (suitable for light loads, heavy loads, rapid acceleration and deceleration, shock loads, vibration loads, etc.), high reliability and long life. At present, a lot of research has been carried out on aero engine bearings, the main bearing type and main parameter design, lubrication method and flanging structure, pocket structure, gasket angle, surface treatment technology, bearing combination use method, etc. have a significant impact on its performance, reasonable design can avoid early failure of bearings, improve its working performance and extend its service life. This paper introduces the particularity of aero engine bearings and sorts out the influencing factors that should be considered in the bearing design process.

Particularities of aero engine bearings

Structural features:

Aero engine bearings: according to the use part, it can be divided into three categories: spindle bearings, accessory transmission gearbox bearings and reducer gearbox bearings.

1. Spindle bearing

Spindle bearings support engine fan rotors, gas generator rotors and power turbine rotors, usually use a combination of three-point angular contact ball bearings and cylindrical roller bearings, three-point angular contact ball bearings bear the axial load and radial load of the rotor, cylindrical roller bearings only bear radial loads, and the working characteristics of spindle bearings are high rotation speed (dn value up to 2.6×106 mm · r/min), high temperature (180 ~ 250 °C, short-term greater than 300 °C), large axial load, short-term light load and overload in the radial direction, axial load change direction and compact installation space. Therefore, spindle bearings have high tolerance levels, generally not lower than P4 level, and also have strict requirements for the machining accuracy of cylindrical rollers; In addition, spindle bearings are mostly lubricated under the ring, which requires corresponding special design.

 

2 Accessory transmission gearbox bearing

 

The bearing of the attachment transmission gearbox supports the gear rotor of the accessory transmission system, and its working characteristics are small load, low temperature (120 ~ 150 °C), and low rotation speed (dn value is not higher than 1.5×106 mm · r/min）。

 

3 Reducer gearbox bearing

 

Reducer gearbox bearing support reducer gear assembly, its working characteristics are large load, low speed and low temperature (120 ~ 150 °C).

 

General requirements

Due to the particularity of the use environment and working conditions of aero engines, the general requirements of bearings are more stringent than those of ordinary bearings, and the relevant requirements are as follows.

 

1. Resistant to high temperature when parking

 

When the engine is running, cold air is introduced to cool the blades, bearings and other parts, and the bearings are also cooled by lubricating oil. However, after the shutdown, these cooling measures do not work, and the turbine heat will be transmitted to the bearing through the bearing housing and shaft, resulting in a rapid rise in the temperature of the bearing in a short period of time, especially the bearing temperature of the engine hot end will rise abnormally, and some engine monitoring data shows that the bearing temperature will reach about 350 °C.

 

2. Lubricating oil interruption running capacity

 

The lubricating oil supply conditions of aero engine bearings are mainly divided into two categories:

1) Before the engine starts to the normal operation of the lubricating oil system, it is in a cold running state, with low speed and small load;

2) When the engine is running, if the lubricating oil system cannot continue to supply oil due to special reasons, and the engine needs to continue to work for a period of time, the bearing will be in a state of poor oil lubrication or even dry operation, and cannot be well lubricated and cooled. To ensure flight safety, bearings need to have the ability to continue to operate normally for a certain period of time without failure without lubricating oil.

 

3 Good workmanship

 

Aero engine bearings need to have good processing, assembly workmanship and lightweight characteristics. In order to reduce the difficulty of assembly, aero engines require as few parts as possible, and the spindle usually needs to be equipped with elastic supports at the support position to adjust the critical speed of the rotor, and oil film dampers are set to suppress rotor vibration, both of which will occupy radial space. In order to save radial space to increase the number of rolling elements and improve the bearing capacity, the bearing outer ring is usually integrated with the squirrel cage elastic support and oil film damper. In order to reduce the number of parts and reduce the difficulty of assembly, the squirrel cage support, mounting flange and oil film damping ring groove are integrated into the outer ring of the bearing, but there are also problems such as increased design difficulty, reduced structural strength, and more stringent material selection and production process.

 

4 Advanced quality assurance system

 

In addition to advanced design concepts and high processing accuracy, it is also necessary to rely on the advanced quality assurance system including process standards, processing equipment, quality inspection, operators, test verification, quality certification and other elements.